Such winches usually have an upright stationary casing, a rotatable drive shaft radially within and extending along the casing, and a winch drum radially outside the casing. In the manufacture of such a winch it is normal to assemble any gearing about the casing before slipping the drum over the upper end of the casing and into place. The drive shaft is inserted (also from the upper end) whenever convenient. In order to hold the drum down, it is usual to fix a winch top assembly to the casing above the drum. The upper end of the drive shaft will extend through a central hole in the winch top assembly so that a drive handle for turning the winch can be connected to it. It is also necessary to hold the drive shaft axially in position. Apart from the general inconvenience of having a piece which can readily come out and get lost or damaged, it would be highly dangerous if the drive shaft slid upwards during winching and ceased to mesh with the drive gear. In some designs of winch at least the presence of the drive shaft is necessary for the operation of the safety ratchet which prevents that drum from rotating in reverse.
It is known to key a circumferential groove in the drive shaft to the casing by inserting keys into the groove through holes in the casing. A ring around the casing stops the keys from coming out of place by a radially outward movement. The ring must in turn be held axially at the level of the keys. (See UK patent specification No. 2061862).